Flocculation is a form of liquid-solid phase separation which facilitates the removal of finely divided particles from a liquid by enhancing agglomeration of suspended particles in order to increase particle size, and is often applied to enable conformity with effluent clarity requirements. Flocculation may be accomplished by chemical means, e.g., addition of a flocculating agent.
Synthetic organic, polymeric, flocculating agents have been utilized in industry since the 1950's. It has been observed, by those skilled in the art, that high molecular weight polymers are particularly useful as chemical flocculating agents, provided that the polymers are water-soluble. Many such water-soluble, high molecular weight, polymeric, flocculating agents are known to those skilled in this art.
Water-soluble polymers have been used as flocculating agents in the treatment of waste sludges, with some degree of success. However, due to modern concerns with environmental protection, it has become increasingly desirable to improve on the performance of conventional flocculants by providing a flocculating agent which increases the rate of settling of suspended solids.
The present invention provides a method for preparing a novel, highly branched, high molecular weight, water-soluble, polymeric flocculant using a polyfunctional monomer such as methylenebisacrylamide, polyethyleneglycol dimethacrylate, n-vinyl acrylamide and the like, as a branching agent. The prior art discloses several attempts to produce high molecular weight, branched, water-soluble polymers. Zweigle, U.S. Pat. No. 4,059,522 discloses the use of branching agents to yield a totally crosslinked system, but flocculants prepared in this manner are insoluble in water and thus ineffective. Morgan, et al., U.S. Pat. No. 3,698,037 discloses branched, cationic homopolymers obtained through incorporation of a multifunctional branching agent in the absence of a molecular weight controlling or chain-transfer agent. It is well known to those skilled in the art, that the molecular weight of such cationic homopolymers is limited, whereas much higher molecular weight polymers may be obtained by copolymerizing cationic monomers with acrylamides. The above discussed patent makes no mention of molecular weight specifics.
Pech, Fr. 2,589,145 discloses a branched copolymer prepared using solution polymerization techniques in the presence of a high activity chain transfer agent. However, the polymers disclosed in this patent have molecular weights below 1 million with solution viscosities of 2200 to 3600 mPa.s at 20 percent polymer concentrations, thus showing these polymers to be truly low molecular weight copolymers. The lowest molecular weight polymer of the present invention is much higher, greater than 1 million, as evidenced by its solution viscosity, (see said above-mentioned copending application.)
Other patent disclosures use shearing of cross-linked polymer chains to obtain desired water solubility. Whittaker, U.S. Pat. No. 4,705,640 discloses the shearing of crosslinked polymer gels which are insoluble in water to physically degrade them to such an extent that they become water-soluble. The preferred method of degradation is mechanical with a high degree of chopping type action, such as subjecting dilute solutions of polymer to rotating blades at up to 20,000 rpm. Degradation is claimed to improve flocculation performance by increasing the effective ionicity of the polymer. The increase in effective ionicity is quantified by measuring the ionic regain (IR); IR=(IAD-IBD)/IAD.times.100 where IAD is the ionicity after degradation and IBD is the ionicity before degradation. The ionicity can be measured by a colloid titration technique as described therein and also Flesher et al, U.S. Pat. No. 4,720,346, which discloses a process for flocculating aqueous suspensions of solids by the use of a polymeric material in the form of small particles rather than a true solution. Flesher et al also disclose the need to shear crosslinked polymer materials, such that the polymer has an ionic regain value of 15 to 70 percent, since polymers having too low an IR value give an inadequate benefit. Flesher et al define shear as that which is applied as an analytical technique to impart properties on polymers, such as IR, so that the polymers may be used in that invention. In Column 11, lines 3-10, Flesher et al further disclose that higher flocculant dose levels are needed to achieve optimum floc stability, sometimes 20 percent more than dose requirements of conventional, water-soluble, polymers. Flesher indicates that branched copolymers can be prepared by using chain transfer agents such as isopropanol and mercaptoethanol in conjunction with cross-liking agents. However, no examples are provided and it appears quite unlikely that Flesher and coworkers discovered compositions corresponding to those claimed herein, which out perform the Flesher and Farrar materials and are simpler to use.
Farrar, in U.S. Pat. No. 4,759,856, also describes, in Column 6, lines 1-6, the need to apply shear to convert crosslinked polymers that would normally have been rejected or that would have been expected to have given poor flocculation properties to a form in which it will give very good flocculation properties. The patentee teaches shearing in such a manner that the polymer undergoes an ionic regain of at least 15 percent, preferably at least 30 percent, and usually at least 50 percent, as a result of the shearing, to effect a conversion to a useful polymer flocculant.
Surprisingly, it has now been discovered that high molecular weight, highly branched, water-soluble, anionic, polymeric flocculants can be produced without the use of high level shear. Polymeric flocculants produced by the practice of the present invention are subjected only to minimal levels of shear, sufficient only to cause solubilization with little or no physical degradation of the polymer. Polymeric flocculants of the present invention outperform the flocculants of the prior art, consistently producing higher settling rates. Additionally, polymeric flocculants of this invention are more convenient and less costly to use than prior art flocculants which require the end user to employ high shear equipment prior to use, in order to gain the desired optimum flocculation effect, thus increasing both the time and cost of the operation.
The present invention enables the preparation of truly water-soluble, highly branched, high molecular weight, anionic polymers particularly useful as chemical flocculating agents. The polymers of this invention are prepared using a branching agent in the presence of a chain-transfer agent to produce a product which is highly branched and water-soluble. The polymers of this invention do not require the application of controlled shear to attain optimum performance, thereby saving additional costs. The present invention has particularly been found useful when applied to branched copolymers comprising acrylamide and at least one ethylenically unsaturated anionic monomer.